Today the state of the art in the area of geotechnical and geophysical investigation is such that velocity profiles for both shear and compressional waves, of the ground, are routinely used in the investigation of construction sites, foundation materials, as well as in the investigation of subsurface earth resources. In-situ dynamic material parameters and elastic moduli of the earth's material can be determined by making use of the velocity data of both shear and compressional waves. For example, regarding geotechnical and geophysical investigation of the earth, such dynamic material parameters include Young's modulus, shear modulus and Poisson's ratio.
In this regard, one principal concern in geophysical investigation of the earth's subsurface is to determine the shear wave velocity. This can be accurately and efficiently accomplished by generating a pure shear wave and then isolating the shear wave from other associated wave forms, such as compressional waves. If the corresponding compressional wave velocity can be measured simultaneously, then it follows that the aforementioned dynamic material parameters can be arrived at by using known relationships involving both shear and compressional wave velocities.
In order to better understanding shear and compressional waves and the distinction therebetween, it may be beneficial to give a closer view towards the same. A compressional wave is an elastic wave and is characterized by longitudinal particle movement during the wave propagation. Also there is an associated elastic volume change. A shear wave on the other hand is characterized by transverse particle movement with no associated elastic volume change. Generally, shear waves travel at a slower velocity than compressional waves, and, therefore, are observed after the compressional wave arrival and in some cases shear waves can be obscured by the compressional wave arrival.
It should be understood that with respect to a shear wave the particle motion occurs in a transverse plane with respect to the direction of propagation, which means that the shear wave can be deemed a vertical shear wave or a horizontal shear wave, depending on the direction of transverse movement.
With respect to the generation of horizontal shear waves, there are methods in use today. For example, one such method involves hitting the end of a log which rests under the front wheels of a parked truck. Another method involves the use of dynamic vane torque devices which supply continuous vibration to the soil. Such continuous vane torque devices do produce good results but the disadvantages of such are that this type of equipment is heavy and complex and includes a great deal of bulky electronic equipment that besides being expensive, is often not practical in remote localities such as densely forested areas.
Therefore, there is a real need for a practical field device that is relatively simple and inexpensive, which can be utilized to determine subsurface shear wave velocities.